Process for forming an electrode

ABSTRACT

A process for forming an electrode utilized in apparatus for minimizing the smear of an image produced by xeroradiography. A sheet comprising a thin layer of dielectric material sandwiched between two X-ray transparent electrically conductive surfaces is cut to the desired outside dimensions. A chemically resistant material is applied to both conductive surfaces in a predetermined pattern. The sheet is dipped into an etching bath removing material from the unmasked portions of the conductive surfaces and then dipped into a neutralizing solution to neutralize the etching chemical. The sheet is then rinsed, dried and the resistant material is removed. An adhesive material is coated on one of the conductive surfaces whereby the sheet, formed into the desired electrode, is affixed to the lid of a cassette utilized in the xeroradiography process

United States Patent Foote 1451 Aug. 12, 1975 Primary Examiner-Carl E. Mall Attorney, Agent, or FirmJames J. Ralabate; Terry J.

[75] Inventor. D. Paul Foote, S1erra Madre, Calif. Anderson; Irving es er [73] Assignee: Xerox Corporation, Stamford,

Conn.

[57] ABSTRACT 22 Filed: Apr. 2, 1973 A process for forming an electrode utilized in appara- [21] APPI' 347,277 tus for minimizing the smear of an image produced by xeroradiography, A sheet comprising a thin layer of 52 US. Cl. 29/2542; 156/3; 156/11; dielectric material Sandwiched between two y 250/315 A transparent electrically conductive surfaces is cut to 51 1m. 01 H0lg 13/00 the desired Outside dimensions- A chemically resistant [58] Field of Search 29/2542, 625; 317/261; material is pp to both Conductive Surfaces in a [56/3, 8 11, 250 5 A predetermined pattern. The sheet is dipped into an etching bath removing material from the unmasked [56] References Cited portions of the conductive surfaces alnd then diriped into a neutralizing solution to neutra ize the etc in UNITED STATES PATENTS chemical. The sheet is then rinsed, dried and the resis 2,673,792 3/1954 GUItOl'l 156/8 tant material is removed An adhesive material is 2 225 222 3x22; 211m-:-------------;;;;;.%f%z 222 any/the 3 265 546 8/1966 Medford 156/3 Sheet formed desred electrode aimed to 3:434:752 12/1969 250/315 A X the lid of a cassetteutilized in the xeroradiography 3,665,570 5/1972 Brooks 29/2542 Process 3,780,288 12/1973 Dryden 250/315 A 4 Claims, 8 Drawing Figures cu'r SHEET MATERIAL TO DESIRED OUTSIDE DIMENSIONS OF SHEET T0 BOTH SIDES DIP INTO c ETCHING LIQUID DIP INTO D neu'mnuzme SOLUTION E RINSE F DRY PATENTEIJIIJBI ems 3.898722 SHEET 1 CUT SHEET A MATERIAL TO DESIRED OUTSIDE DIMENSIONS APPLY CHEMICALLY B RESISTANT MATERIAL TO BOTH SIDES OF SHEET C 0.. mm F/G.

ETCHING LIQUID DIP INTO D NEUTRALIZING SOLUTION E RINSE F DRY REMOVE APPLY ADHESIVE G CHEMICALLY MATERIAL TO RESISTANT ONE SIDE OF MATERIAL SHEET PATENTED AUG 1 2 i975 SHEET PROCESS FOR FORMING AN ELECTRODE BACKGROUND OF THE INVENTION Image smear, due to ion'caused undercutting, is a nonfaithful representation of the inner structure of a test object manifested as a loss of detail in image areas of a xeroradiographic reproduction. Image smear oc' curs primarily in those low contrast regions of the image which receive the largest amount of ray energy. Undercutting occurs at an edge between regions of high and low X-ray exposure.

It is well known that the longer wavelength X-rays are efficient in ionizing the air near the surface of a xerographic photo-emitting receptor during exposure. The amount of air ionization is proportional to the intensity of the X-rays reaching the photoreceptor surface. The mechanism of ionization of the air includes the direct photoelectric action of both X-ray photons on the air and the secondary photo-injected electrons from the photoreceptor on the adjacent air molecules. Both positive and negative ions are formed in the process. If these ions are allowed to diffuse randomly, they will partially discharge the electrostatic image on the photoreceptor in a manner which tends to destroy rather than enhance the latent image of the object being examined. Xerographic undercutting is caused by the ionization of air above the surface of the xeroradiographic plate. In particular, during exposure to a specimen of differing thicknesses, the charged plate is discharged more completely at the thinner sections, causing discontinuities in the charge pattern. Across these discontinuities, strong localized fields curve sharply from one charge concentration to the other. The ions having a polarity opposite to the charge on the photoreceptor are attracted by the local fields, and are deposited on and discharge the more highly charged sides of voltage discontinuities. As the exposure continues, and the edges become discharged by the action of the ions, the field pattern moves inward toward the center of the more highly charged area, additional ions will follow, destroying more and more of the image.

The technique of minimizing ion undercutting in xeroradiography by positioning a biased electrode in spaced proximity to the photoreceptor surface is shown by Vyverberg US. Pat. No. 2,802,948. As set forth therein, the electrode is biased in the range from 700 to 1,800 volts, preferably 1,200 volts, by a DC. voltage source.

In the xeroradiographic system described in copending application Ser. No. 874,834, filed on Nov. 7, 1969, now US. Pat. No. 3,650,620, a photoreceptor is automatically charged and inserted into a cassette unit. The cassette unit, forming a light-tight environment for the enclosed photoreceptor, may be transported to an exposure station whereat the object to be examined is exposed to X-rays. To minimize ion undercutting, the system disclosed by Vyverberg requires the aforementioned voltage source to generate an appropriate electric field whereby negative ions are prevented from dis charging the latent electrostatic image. The portability of the exposure system disclosed in Vyverberg is therefore severly limited by the weight and bulkiness of the required voltage source.

SUMMARY OF THE INVENTION The present invention provides a technique for forming an electrode which is utilized to minimize image smear, due to ion undercutting, in xeroradiography. A thin layer of dielectric material sandwiched between two X-ray transparent electrically conducting surfaces is cut to the desired outside dimensions. A chemically resistant material is applied to both conductive surfaces in a predetermined pattern. The sheet is dipped into an etching bath removing material from the unmasked portions of the conductive surfaces and then dipped into a neutralizing solution to neutralize the etching chemical. The sheet is then rinsed, dried and the resistant material is removed, thereby forming the electrode which functions as a distributed capacitor. An adhesive material is coated on one of the conductive surfaces. A charged photoreceptor is enclosed in a light-tight cassette unit and the formed electrode is affixed to the lid of the cassette unit by means of the adhesive material. The photoreceptor, comprising a photoconductor overlying a conductive substrate, is supported adjacent the cassette bottom. The distributed capacitor is initially charged by a voltage source which is then removed. The object to be examined is exposed to Xrays and a latent electrostatic image is formed on the photoconductor surface. The distributed capacitor retains the charge initially applied thereto for a substantial time period, thereby enabling a plurality of exposures to be made with minimization of image smear without recharging the capacitor.

It is an object of the present invention to provide a novel method for forming an electrode for utilization in xeroradiography whereby image smear, or ion undercutting, is minimized.

It is a further object of the present invention to minimize image smear, or ion undercutting, in a xeroradiographic system wherein the charged photoreceptor is enclosed in a light-tight cassette unit.

It is still a further object of the present invention to provide a novel method for forming an electrode utilized in xeroradiography and comprising a thin, layer of dielectric sandwiched between two X-ray transparent electrically conductive surfaces, the formed electrode being affixed to the lid of the cassette. In the operating position, one of the conducting surfaces is positioned adjacent the photoreceptor surface and the other conducting surface is coupled to the photoreceptor substrate, the field produced by the capacitor being sufficient to minimize image smear and ion undercutting.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention as well as other objects and features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates the method steps of the present invention;

FIGS. 2 and 3 are schematical representations illustrating the electrode formed by the novel method of the present invention; and

FIGS. 4-8 illustrate the embodiment shown in FIGS. 2 and 2 as utilized in a cassette which encloses a sensitized photoreceptor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, the series of steps (A) (H) required to produce the novel electrode of the present invention is schematically represented.

In step (A), the starting material, comprising a thin dielectric sandwiched between two X-ray transparent electrically conducting surfaces, is cut to the desired outside dimensions. Typically, the starting material may be Mylar, comprising polyethylene tetraphthalate, a trademark of E. I Dupont de Nemours, Inc., 0.003 inches thick, aluminized on both sides to a thickness in the range from about 100A to about 1,500A. Alumi nized Mylar may be obtained from the Hy-Sil Manufacturing Company, Revere, Mass. Other materials which may be utilized as the conducting surfaces include copper, brass, gold, etc. An example of another dielectric material includes Teflon, comprising tetraflouroethylene, also a Dupont trademark. The starting material, regardless of the material components utilized, is preferably in the sandwich form described hereinabove.

In step (B) a chemically resistant material (resistant to the etching material utilized in step (C) is applied to both sides of the Mylar in the desired pattern. A typical chemically resistant material which has been successfully utilized in the present invention comprises Avery Flex K-l labels, manufactured by the Avery Products Corporation, San Marino, Calif.

In step (C), the Mylar sheet is dipped into an etching material, such as sodium hydroxide solution, at room temperature for approximately seconds. This procedure removes the aluminum from the unmasked portions of the sheet, thus producing the desired electrode pattern.

Other etching materials which may be utilized include potassium hydroxide and hydrofluoric acid.

Different etchants may have to be utilized if the conducting surfaces are made from material other than aluminum.

The Mylar sheet is then dipped in a weak acid solution, such as 10% acetic or tartaric acid, to neutralize the sodium hydroxide as shown in step (D).

In step (E) the Mylar sheet is rinsed with water and in step (F) the sheet is dried by any conventional technique. After the sheet is completely dried, the chemically resistant material is removed from both conducting surfaces and an adhesive is applied to one conducting surface, as set forth in steps (G) and (H). An adhesive material which has been utilized successfully in the present invention is a transfer tape (Avery Type N-lO) manufactured by the Avery Company. At this point, the formed electrode may be mounted to the xeroradiographic cassette lid.

FIG. 2 is a plan view of an electrode formed in accordance with the teachings of the present invention and FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2. Dielectric 18 is shown sandwiched between conducting surfaces and 22 (shown in dashed lines). Conducting surfaces20 and 22 are offset to allow for electrical connections to both surfaces. Typical dimensions (in inches) of the electrode are as follows:

Length of dielectric l8 (Dimension 21) 14.62 Width of dielectric l8 (Dimension b) 9.30 Thickness of dielectric 18 0.003 Length of conducting surfaces 20 and 22 (Dimensions C and C 14.00 Width of conducting surfaces 20 and 22 (Dimension d) 9.00 Thickness of conducting surfaces 20 and 22 12501 250A Offset between conducting surfaces 20 and 22 (Dimension e) 0.25

Referring now to FIG. 4, there is shown a schematical representation of apparatus utilizing the electroded cassette formed in accordance with the teachings of the present invention. the xeroradiographic plate or photoreceptor 10 comprises generally a layer of an insulating photoconductor 12, such as vitreous selenium, overlying a conductive metal backing plate 14. As is well known in the xerographic art, layer 12 has the property of retaining charge in the dark and of discharging or dissipating its retained charge when exposed to light, X-rays, gamma rays or other penetrating radiation.

Spaced apart from, for example, within 1 cm, and substantially parallel to plate 10, is element 16 comprising a thin layer of dielectric material 18 sandwiched between two X-ray transparent, electrically conductive surfaces 20 and 22 and formed in the manner described hereinabove. Other materials may be utilized as the conductive surfaces and dielectric as set forth previously. Conducting surface 20 is electrically connected to metal backing plate 14 via resistor 24. An electric field perpendicular to the surface of the photoconductor 12 is generated in the region between the xerographic plate 10 and conducting surface 22 due to the initial charge which is utilized to sensitize the surface of photoconductor 12. The electric field in this region is further enchanced by placing an electrostatic charge on conducting surfaces 20 and 22. Dielectric material 18 and conducting surfaces 20 and 22, the electrode formed in accordance with the teachings of the present invention, functions as a distributed capacitor which is charged by connecting voltage source 28 across the conducting surfaces. Voltage source 28 is illustrated in a manner to indicate that after it charges conducting surfaces 20 and 22, it is removed from contacts 25 and 27 prior to the exposure of object 26 by the X-ray source 29.

As set forth hereinabove, the thickness of dielectric layer 18, when formed of Mylar, is about .003 inches whereas the conducting surfaces 20 and 22, when formed of aluminum, are approximately 0.0005 inches thick. The value of resistor 24 is typically 10 ohms.

In operation, DC. voltage source 28 is connected across conducting plates 20 and 22 through resistor 24 and applies a potential thereacross in the range from about 400 volts to about 1,600 volts, depending on the initial charge placed on photoconductive layer 12, and of .a polarity as shown such that a negative charge is formed on conducting surface 22. It is assumed that the surface of photoconductor 12 has initially been charged to a positive potential. Object 26 is then exposed to penetrating radiation generated by X-ray source 29. The voltage source 28 may then be disconnected from conducting plates 20 and 22 or, alternatively, may be disconnected prior to exposure.

The electric field perpendicular to the surface of photoconductor 12 tends to confine the air ions to the immediate region where they are generated. Ions charged to the same polarity as the polarity of the elec' trostatic charge on the photoconductor surface are attracted to the conducting surface 22 while the ions charged to a polarity opposite to that of the electrostatic charge on the surface of the photoconductor tend to further discharge the photoconductor 12 in the same pattern as the X-rays which expose the photoconductor 12. Consequently, the image of the object being examined is improved due to the presence of the electric field generated by the capacitor 16 which is maintained even though the plate is discharged by the exposure. The electric field tends to maintain ions in the position over the plate where they were formed (unless they have excess lateral energy from their formation). Negative ions are forced toward the plate contributing constructively to the exposure while positive ions are forced away from the plate. The distributed (sheet) capacitor 16 does not need a permanent charging source and may be charged, as set forth hereinabove, just before exposure. Experience with modest energy X-ray (in the range from about 25 KVP to about 45 KVP) exposures shows that capacitor 16 will maintain its charge for at least 50 exposures.

It should be noted that if photoconductor layer 12 is initially negatively charged, the charge applied to conducting surfaces 20 and 22 should be reversed by reversing the connections of voltage source 28, i.e. a positive charge is formed on surface 22. In this manner, minimization of image smear is also accomplished regardless of the polarity of the initial charge on the surface of photoconductor 12.

Since capacitor 16 is chosen to be transparent to X-ray radiation, the X-ray radiation does not affect the charge thereon.

In practice, it has been found that the sum of the charge on photoconductive layer 12 and the charge placed on the distributed capacitor 16 should be in the range of about 400 volts to about 2,000 volts. The plates are normally charged in the range from about 400 volts to about 1,600 volts. For illustrative purposes, if the plate is charged to 400 volts, the potential applied to the capacitor may be 1,600 volts.

Resistor 24 is not necessary for the operation of the present invention. However, resistor 24 acts to prevent the thin aluminum surfaces from burning off the electrodes 20 and 22 around the contact area (illustrated in FIGS. 58 hereinbelow) during the charging of capacitor 16 or if conducting surface 22 is accidently brought into contact with photoconductive layer 12. In addition, resistor 24 protects against shock hazard when used, for example, in the cassette illustrated in FIGS. 4-8. In lieu of resistor 24, substrate 14 may be connected directly to conducting surface 20 or via a resistor of smaller resistance value.

FIGS. 5-8 show views of the embodiment shown in FIG. 4 when utilized in a cassette unit of the type described in copending application U.S. Ser. No. 208,973 filed Dec. 16, 1971. The details of the cassette shown in the copending application are not set forth herein since the inventive concept of the present invention set forth hereinabove may be utilized in any X-ray imaging cassette. Therefore, the figures shown are highly schematical views of the electrode formed in accordance with the teachings of the present invention when utilized in combination with a cassette.

FIG. 5 is a plan view of a cassette unit 30, in the closed position, shown in a transparent form to illustrate the capacitor 16 clearly. Conducting surface 20 is connected to a ohms deposited resistor 24 (a lumped resistor may be utilized instead) via conductor 32. Conductor 32 may comprise a conducting tape formed of aluminum with a conductive adhesive, a deposited aluminum film, copper tape, etc.

FIG. 6 is a front elevation of the cassette 30 showing the substrate of photoreceptor plate 10 connected to conducting surface via conductor spring 34, spring contact 36 and deposited resistor 24. Also shown are guide rails 35 for supporting the photoreceptor plate within the cassette.

FIG. 8 is a side elevation of cassette 30 further illustrating the connection of conducting surface 20 to the substrate 14 of plate 10 via conductor spring 34.

FIG. 7 is a plan view of conducting surface 22 with contact surface 38 for conductor spring 34. Contact 25 is connected to conducting surface 22 via a strip of conducting tape 29 as shown. Contact 27 is connected to contact surface 38 via a thin conducting strip 31.

The formed electrode, or distributed capacitor 16, is placed inside the cassette lid and held thereto by a suitable adhesive with the top conducting surface 20 electrically connected to the metallic substrate of plate 10 as shown.

The present invention provides an additional advantage to those discussed hereinabove. If the cassette material is made of plastic, or other insulating material, a surface electrical charge may be formed in various patterns on different portions of the cassette. This will lead to a non-uniform electric field above the surface of photoreceptor 10 which is usually detrimental to the image to be formed and reduces the possibility of image repeatability. The utilization of the distributed capacitor 16 in the manner described hereinabove substan tially eliminates this problem by providing a uniform electric field of a magnitude which masks the effect of the non-uniform fields.

While the invention has been described with reference to its preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.

What is claimed is:

l. A method for forming an electrode for use in a cassette for enclosing in a light-tight environment a photoconductor layer formed on a conductive substrate, said photoconductor layer having a latent electrostatic charge formed thereon, said cassette including lid and bottom portions, said photoconductor layer being supported in the bottom portion and said electrode being affixed to the lid portion of said cassette, said method comprising the steps of:

a. cutting a starting material comprising a layer of dielectric material sandwiched between two electri- Cally conductive surfaces to a predetermined size such that the starting material may be affixed to said lid portion,

b. applying an etchant resistant material to selected portions of both conductive surfaces, said selected portions corresponding to a desired electrode pattern,

c. subjecting the starting material to an etchant whereby the non-selected portions of the conductive surfaces are removed from the starting material,

d. removing the starting material from the etchant,

e. removing the etchant resistant material from said starting material, and

f. applying an adhesive material to one of the conductive surfaces and affixing the starting material to said cassette lid.

2. The method as defined in claim 1 further including conductive surfaces are X-ray transparent. the steps of neutralizing the etchant remaining on the v 4. The method as defined in claim 1 wherein said distarting material subsequent to step (D), and then rinselectric material comprises Mylar and said conductive ing and drying the starting material. surface comprise aluminum.

3. The method as defined in claim 1 wherein said 

2. The method as defined in claim 1 further including the steps of neutralizing the etchant remaining on the starting material subsequent to step (D), and then rinsing and drying the starting material.
 3. The method as defined in claim 1 wherein said conductive surfaces are X-ray transparent.
 4. The method as defined in claim 1 wherein said dielectric material comprises Mylar and said conductive surface comprise aluminum. 